The present invention relates to a plate material used to manufacture an intaglio by a laser plate-making system.
Recently, as it becomes popular to carry out a desktop publishing with some proper equipments, such as a word processor, a personal computer or the like, various kinds of printing methods with high speed and high resolution are proposed. However, these proposed printing methods are encountered by the problem that they are not suitable for the mass printing and that it is impossible to print a full-color image with an excellent gradation.
On the other hand, a printing method suitable for mass printing of a full-color image with excellent gradation includes a gravure printing, for example. An intaglio used in the gravure printing is generally a metal intaglio manufactured by plate making methods, such as a conventional gravure method, an inverted halftone gravure method, a mechanical engraving method or the like. The intaglio usually has a hard chromium-plated layer provided on its surface and hence becomes resistant to scratch and abrasion and high in durability. However, when the intaglio is manufactured, manufacturing processes thereof are complicated and manufacturing equipments therefor are large-sized. Therefore, the gravure printing is encountered by the problem that a printing plate cannot be manufactured with ease within a short period of time and that it costs much to print only a small amount of copies.
In view of such problems, there has been proposed a printing method in which a printing plate that allows the full-color image with excellent gradation to be printed can be manufactured with ease and which is suitable when a small or large amount of copies are to be printed, i.e., a method (U.S. Pat. No. 5,126,531; DIGIC, 30 (published by SONY CORP in January, 1994)) in which an intaglio made of thermoplastic resin is manufactured by a laser plate-making system and used for the gravure printing. According to the laser plate-making system, a laser beam is radiated on a plate sheet made of the thermoplastic resin and a radiated portion thereof is melted, decomposed, burned or sublimated to form concavities with a depth of about 8 .mu.m on a surface of the plate sheet. In this laser plate-making system, when the laser beam is radiated, if a pulse width of the laser beam or an intensity thereof is modulated in response to a gradation of an image to be formed, then an intaglio made of resin which can be used for the printing with gradation is manufactured. Accordingly, according to the laser plate-making system, it becomes possible to manufacture the intaglio with lower costs in more simplified processes within a shorter time as compared with general gravure plate making methods, such as the conventional gravure method, the inverted halftone gravure method, the mechanical engraving method or the like.
For such laser plate making, a plate material has been proposed (in Japanese Published Patent Publication No. 246165/1993) which is compounded of some materials, such as a thermoplastic resin, a carbon black as a light absorbing agent for absorbing laser light and changing it to thermal energy, a curing agent for improving solvent resistance and abrasion resistance of the plate sheet, or the like.
When the gravure printing is carried out with the intaglio made of such plate material by the laser plate-making system, as shown in FIG. 1, an intaglio 12 is wound around a printing cylinder 11 and a water-base or oil-base ink 14 stored in an ink fountain 13 is supplied to a surface of the intaglio 12 by an ink roller 15. Any unnecessary ink is removed from the surface of the intaglio 12 by a doctor blade 16 to leave the ink 14 only in concavities 17 formed on the surface of the intaglio 12. Then, the ink 14 left in the concavities 17 is transferred to a printing paper 19 conveyed by a feeding roller 18. Thus, it is possible to print the full-color image with excellent gradation in houses and offices.
However, the resin intaglio made of a plate material for laser plate making disclosed in Japanese Laid-open Patent Publication No. 246165/1993 has disadvantage that its surface hardness is remarkably low as compared with the hard chromium-plated metal plate. Therefore, when comparatively large pigment particles contained in the ink or a talc and a clay detached from the printing paper are pressed by the doctor blade against the surface of the intaglio, linear scratches with depth (about 2 to 3 .mu.m) of an order approximate to a plate depth (about 8 .mu.m) are made as the plate material is used repeatedly. The ink enters the scratches and causes scumming and linear scumming on a printed matter to thereby lower printing quality. To improve this disadvantage, it is demanded to increase the number of copies which can be printed with one printing plate without the printing quality being lowered.